The proliferation of microorganisms and the resultant formation of slime is a problem which commonly occurs in aqueous systems. Problematic slime producing microbes may include: bacteria, fungi, and algae. Slime deposits typically occur in many industrial aqueous systems including cooling water systems, pulp and paper mill systems, petroleum operations, clay and pigment slurries, recreational water systems, air washer systems, decorative fountains, food, beverage, and industrial process pasteurizers, sweetwater systems, gas scrubber systems, latex systems, industrial lubricants, cutting fluids, etc.
Biocides and antimicrobials are used to control or eliminate microbial growth in a number of different aqueous media. If left untreated, microbes and microbial biofilms (slimes) can cause deterioration of cooling tower structures, loss in heat exchange efficiency in a cooling system, aesthetic defects in decorative fountains, promotion and acceleration of corrosion on metal surfaces, increased down time, or breaks in paper sheets in pulp and paper systems. Bacterial slimes may also be objectionable as they relate to cleanliness and sanitation in breweries, dairies, and other industrial food and beverage process water systems. The proliferation of microbial contamination in lubricants and cutting fluids is a common problem due to the elevated temperatures and unsanitary conditions found in many metal working plants. As a consequence of the deleterious effects of uncontrolled microbial growth and contamination in many industrial processes, different antimicrobials have been developed to aid in eliminating and controlling microbial growth.
Often, one antimicrobial (biocide) is insufficient to control microbial growth in the aqueous media. Biocides may act in combination, i.e. synergistically, to yield better antimicrobial performance as opposed to the efficacy obtained when each biocide is used separately. Biocides may act on the target microbe in a number of different ways to cause cell stress or death. The mechanisms by which biocides exert antimicrobial activity depend upon a number of factors which include the chemical properties of the antimicrobial, and the biochemical and physical characteristics of the target microbe. Some biocides target the cell membrane or cell wall. Others target critical enzymes or the cellular metabolic machinery which leads to cell death or disruption of cellular replication.
The combination of two biocides may yield enhanced efficacy beyond the cumulative or additive effect of the two biocides. This likely reflects a synergistic anti-microbial effect on some essential component(s) of the cell for survival and sustained growth. A combination of two biocides that are synergistic allows for the addition of lesser amounts of the individual biocides to achieve the desired level of microbial control. This has both advantageous environmental and economic impacts. It allows for reduced discharge of potential environmental pollutants and a more cost effective control program for these diverse industrial systems.
For this invention, the methylchloro/methylisothiazoline biocide is a broad spectrum antimicrobial agent that is widely used in industrial systems to control algae, bacteria, and fungi. Commercial preparations of the compound for use as a cooling tower biocide (KATHON.RTM. WT; a microbial control agent available from Rohm and Haas Company, Philadelphia, Pa.) are water-based formulations of inorganic stabilizers and active ingredients. The active ingredients, 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl -4-isothiazolin-3-one, are present in the approximate ratio of 3:1.
Isothiazolones are effective against many microorganisms. The microbiocidal activity of isothiazolones is likely due to their electrophilic nature and their interaction with certain key enzymes such as cellular dehydrogenases as well as the pronounced effect they have on cellular respiration.
It was shown in previous research that isothiazolones can be used to achieve significant disinfection of a bacterial biofilm, but it is not a good agent to remove adherent biofilm. Conversely, stabilized sodium hypobromite may act as a penetrant, and disrupt adhered biomass. Therefore, the persistence of isothiazolones coupled with the reactivity and the biofilm removal properties of stabilized sodium hypobromite yields an antimicrobial composition with superior performance compared to the results obtained when either biocide is used independently. Also, the combination of these two biocides clearly exerts an enhanced (synergistic) antimicrobial effect on planktonic microorgansims as indicated in the data shown herein. The exact mechanism for this observed synergy remains unknown.
It is an object of the present invention to provide novel antimicrobial compositions which provide enhanced effectiveness for controlling or inhibiting the growth of microorganisms in aqueous systems. It is another object of this invention to provide an improved method for controlling microorganisms in aqueous systems. It is an advantage of the present invention that the biocidal compositions permit a reduction in the amount of biocide required to achieve acceptable microbiological control.
Important applications of the synergistic antimicrobial compositions of the present invention include but are not limited to inhibiting the growth of bacteria and fungi in aqueous media. The composition of the present invention possesses unexpected synergistic activity against microorganisms, including bacteria and fungi.
Stabilized sodium hypobromite is less volatile and more stable than other halogenated molecules such as sodium hypochlorite and sodium hypobromite. Also, much higher levels of available halogen for microbial disinfection are attained using stabilized sodium hypobromite than with other halogenated antimicrobials. Bromate formation is significantly reduced with the use of stabilized sodium hypobromite (1997, Dallmier, A. W. and W. F. McCoy. PCT Int. Appl., WO 97/34827). The United States EPA identified some health concerns relevant to bromate formation (1995, Amy, G. et al. Water Supply. 13(1):157). Animal carcinogenesis has been linked to low bromate levels in drinking water (1995, Fawell, J. K. and G. O'Neill. Water Supply. 13(1):29). Further, stabilized sodium hypobromite yielded reduced generation of adsorbable organic halogen (AOX) in laboratory studies and process waters.
Stabilized sodium hypobromite yields significant reduction of viable microbial populations at concentrations between 1.0 and 2.0 ppm total residual oxidant (as chlorine). The isothiazolone mixture (1.5% active ingredients) is typically used at concentrations of 100 to 200 ppm as product to obtain similar reductions in microbial populations. This invention provides superior microbiological control by combining stabilized sodium hypobromite with a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one. The combination of the two antimicrobials allows for significantly less use of either antimicrobial compared to the necessary amount of each individual antimicrobial to achieve the same biocidal performance.
As is well known in the art, isothiazolines exhibit synergistic antimicrobial properties when combined with certain other biocides. Synergistic blends of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one with glutaraldehyde were disclosed in U.S. Pat. No. 4,539,071; with dodecyl guanidine hydrochloride disclosed in U.S. Pat. No. 4,661,503; and with 2-(thiocyanomethylthio)-benzothiazole disclosed in U.S. Pat. No. 4,595,691. The Pocius reference (U.S. Pat. No. 4,295,932) discloses a synergistic composition of isothiazolones with either chlorine or chlorine dioxide. Microbiocidal compositions containing halogen-releasing compounds are disclosed in WO 96/14092. However, the synergistic biocidal composition described herein was not disclosed or suggested by the above-mentioned references.